* Create `src/config.h` instead of passing everything on the compiler
command line.
* To reduce the magnitude of this change, add `--include src/config.h`
to CFLAGS instead of adding `#include "src/config.h"` at the top of
each source file. Not all compilers support this, but I don't think
we care about anything other than gcc and clang at this point.
* Rather than generate `src/config_opts.inc`, emit `JQ_CONFIG` directly
into `src/config.h`.
* Rather than generate `src/version.h`, define `JQ_VERSION` as an alias
for the standard `PACKAGE_VERSION`.
Dag-Erling Smørgrav